Sintered ferrite magnets having magnetoplumbite-type (M-type) structures are used in various applications including motors, rotors of electric generators, etc. Sintered ferrite magnets having higher magnetic properties are recently required for the purpose of reduction in size and weight of motors for automobiles and increase in efficiency of motors for electric appliances. Sintered ferrite magnets used in motors for automobiles, for instance, are required to be thin for the purpose of reduction in size and weight. That is, demand is mounting for sintered ferrite magnets having high Br, as well as such high Hcj and squareness ratio (Hk/Hcj) that their magnetization is not reduced by a demagnetization field generated when they are made thinner.
M-type sintered ferrite magnets such as Sr ferrite or Ba ferrite, etc. have conventionally been produced by the following steps. An iron oxide and a Sr or Ba carbonate, etc. are mixed and calcined to produce calcined clinker by a ferritization reaction. The calcined clinker is coarsely pulverized, and a predetermined amount of the resultant coarse powder is charged into a fine pulverizer, together with SiO2, SrCO3, CaCO3, etc. for controlling the sintering behavior, and further Al2O3 or Cr2O3 for controlling Hcj, if necessary. Wet fine pulverization is conducted in a solvent until their average diameter becomes 0.4-1.2 μm. A slurry containing the resultant fine ferrite particles is molded under pressure while orienting the fine ferrite particles in a magnetic field. The resultant green body is dried and then sintered, and finally worked to a desired shape.
The addition of Al2O3 or Cr2O3 improves Hcj but drastically reduces Br. This phenomenon occurs because Al3+ or Cr3+ dissolved in the M phase acts to reduce saturation magnetization σs, and suppress grain growth during sintering.
To solve this problem, Japanese Patent 3,337,990 (corresponding to U.S. Pat. No. 6,139,766) proposes a sintered ferrite magnet comprising ferrite with a hexagonal structure as a main phase, which has a composition represented by A1−xRx (Fe12−yMy)zO19, wherein A is at least one element selected from the group consisting of Sr, Ba and Pb, Sr being indispensable, R is at least one element selected from the group consisting of rare earth elements including Y, La being indispensable, M is Co or Co and Zn, and x, y and z meet the conditions of 0.04≦x≦0.6, 0.04≦y≦0.5, and 0.7≦z≦1.2. According to the description in Example 1 of Japanese Patent 3,337,990, this sintered ferrite magnet is produced by formulating a mixture of Fe2O3 powder, SrCO3 powder, Co3O4 powder and CoO powder with La2O3 powder, and further with 0.2% by mass of SiO2 powder and 0.15% by mass of CaCO3 powder, and then calcining, pulverizing, and molding and sintering in a magnetic field. This production step is called “prior addition method,” because La (R element) and Co (M element) are added before calcining. The resultant sintered ferrite magnet has high Hcj and Br (Sample Nos. 11-14). However, Sample Nos. 11-14 have as low squareness ratios Hk/Hcj as 77.6-84.1%. Accordingly, to meet the above requirement of further thinning, magnetic properties should be improved. In addition, it should be noted that an extremely small amount of CaCO3 is added in a mixing step before calcining in Sample Nos. 11-14.
Japanese Patent 3,262,321 (corresponding to U.S. Pat. No. 6,086,781 and U.S. Pat. No. 6,258,290) discloses a method for producing a hexagonal sintered ferrite magnet having a composition comprising 1-13 atomic % of an A element (at least one element selected from the group consisting of Sr, Ba and Ca, Sr or Ba being indispensable), 0.05-10 atomic % of an R element (at least one element selected from the group consisting of rare earth elements including Y, or including Bi), 0.1-5 atomic % of an M element (Co or Co and Zn), and 80-95 atomic % of Fe, the method comprising adding compounds containing Co and/or the R element to particles comprising hexagonal ferrite containing at least the A element as a main phase, or further adding compounds containing Fe and/or the A element, and then molding and sintering. This method is called “post-adding method,” because the R element and the M element are added in a pulverization step after calcining. Sintered ferrite magnets obtained by this method, however, fail to sufficiently meet the requirement of thinning, needing further improvement in magnetic properties, as is clear from Sample Nos. 1 and 2. In addition, it should be noted that an extremely small amount of CaCO3 is added in a mixing step before calcining in Sample Nos. 1 and 2.
Japanese Patent 3,181,559 (corresponding to U.S. Pat. No. 6,402,980) discloses a sintered ferrite magnet comprising hexagonal ferrite as a main phase, and having a composition represented by the general formula: Ca1−xRx(Fe12−yMy)zO19, wherein R is at least one element selected from the group consisting of rare earth elements (including Y) and Bi, La being indispensable, M is Co and/or Ni, and x, y and z meet the conditions of 0.2≦x≦0.8, 0.2≦y≦1.0, and 0.5≦z≦1.2. This sintered ferrite magnet, however, has as low Hk/Hcj as 75.9-80.6% (see Sample Nos. 21-23), failing to meet the above requirement of thinning. Also, as shown in FIGS. 15 and 16 in Japanese Patent 3,181,559, when x=0.4 or more in the composition of CaxSr(0.4−x)La0.6Co0.6Fe11.4O19 (x=0, 0.2, or 0.4), the magnetic properties tend to become low. This appears to be due to the fact that the Co content is as very high as 0.6.
JP11-224812A discloses a sintered ferrite magnet having both an M-type ferrite phase and a spinel ferrite phase, the M-type ferrite phase comprising 1-13 atomic % of an A element (at least one element selected from the group consisting of Sr, Ba, Ca and Pb, Sr and/or Ca being indispensable), 0.05-10 atomic % of an R element (at least one element selected from the group consisting of rare earth elements (including Y) and Bi), 0.1-5 atomic % of an M element (bivalent metal element such as Co, Zn, Mg, Mn, Cu, etc.), and 80-95 atomic % of Fe. However, this sintered ferrite magnet has poor magnetic properties because of having both the M-type ferrite phase and the spinel ferrite phase.